More thoughts.

Having the paperwork for a Designated Primary Caregiver, in essence gives the holder the right to possess Medical Marijuana.
I don't think the wording of CHAPTER 69.51A RCW , is making this easily available to the public.
All I would have to do is designate all my friends, then they would have paper stating they may possess Medical Marijuana.

RCW 69.51A.010 Definitions.
(2) ā??Primary caregiverā? means a person who: (NOT PEOPLE)

Since there is no Designated Primary Caregiver registery :
Having 3 caregivers would pose no problems unless, 2 or more were asked for their papers at the same time.